This application claims priority to German Application 10210326.7 filed 08 Mar. 2002.
The invention concerns a magnetic measuring structure with portions which are magnetised alternately in the longitudinal direction, referred to herein for the sake of brevity as a xe2x80x98measuring scalexe2x80x99, for a length measuring apparatus, and a process for the production thereof.
When arranged on the periphery of a cylinder such a length measuring apparatus can naturally also be used basically for angle measurement purposes.
The length measuring apparatus includes on the one hand a measuring scale on which the units of length are recorded and a sensor unit which is moved relative to the measuring scale in the measuring direction. Generally in that case the arrangement registers the number of units of length which are covered on the part of the sensor unit in that relative movement, that is to say how many units of length the sensor unit entirely or partially travels over. The absolute position at the end of the relative movement can only be calculated if the starting position, prior to the relative movement, is known.
For that purpose the straight or curved measuring scale has respective codings, generally uniform and periodic codings, which are arranged in succession in the measuring direction, only in one single track or in a plurality of tracks in mutually juxtaposed relationship, with the pitch spacing being different from one track to another. In addition, provided along the measuring distance, in general mostly only at a single lengthwise position, there is a reference mark, the position of which represents the absolute zero position and over which therefore the sensor unit first has to travel once, for setting the apparatus in operation, in order thereby to predetermine an absolute start value.
In addition however length measuring apparatuses which provide for absolute measurement are also known. In that case, by virtue of the design configuration of the measuring scale divisions and the evaluation procedure involved for example by just once setting the sensor to any location on the measuring scale it is possible directly to ascertain the absolute position of the sensor on the measuring scale without relative displacement of the sensor with respect to the measuring scale and without the need to initially move the sensor to a reference point on the measuring scale.
Irrespective of whether the system involves an incremental or an absolute length measuring system, the measuring apparatus according to the invention includes one or more or a multiplicity of magnets which implement modulation of the signal to be detected. For example the individual units of length are applied in sequence in the measuring direction to the measuring scale, in the form of different magnets or magnetisations, by way of example in the form of segments which are each of equal length in the measuring direction and which involve alternate poles.
The sensor unit which is moved relative thereto in the measuring direction and which, besides the actual sensor, generally already includes at least parts of the electronic evaluation system, detects the magnetic field which constantly alters in the measuring direction, as an analog signal in the form of a sinusoidal oscillation or a sine-like but uniform oscillation. A substantial advantage of this method is the fact that the sensor can be moved at a spacing relative to the measuring scale, that is to say in a contactless mode. The measuring scale and also the sensor are thus not subjected to any mechanical wear. In addition there is only a limited need for parallelism of the direction in which the sensor is guided relative to the direction in which the measuring scale extends.
In particular the spacing between the sensor and the measuring scale which should be at about 1.0 mm may also change somewhat.
The measuring scale may have one or a plurality of mutually juxtaposed tracks which each have the magnetised segments of alternate poles.
Thus, the one track, as a pure counting track, can be equipped with regular sequences of magnetisation portions while the other track serves to mount reference marksxe2x80x94in punctiform or region-wise mannerxe2x80x94, that is to say reference marks for the purposes of marking for example the absolute zero point, the end of the measuring scale or the like. Such a reference track may be non-magnetised or additionally regularly magnetised in the lengthwise region between the reference marks.
Likewise, absolute encoding of the measuring structure can be effected by means of a plurality of mutually juxtaposed tracks, in which case then the length of the differently poled magnetised segments on the individual tracks is mostly of different magnitude.
Hereinafterxe2x80x94without however limiting the invention theretoxe2x80x94consideration will be based on the specific case where both the regular sequences of magnetisations for registering a distance covered and also at least one irregular sequence of magnetisations for producing reference marks are present on one and the same track. For that purpose, the width of the track can also be divided into two narrow sub-tracks, in the region of the irregular sequences.
In that respect, the problem which arises is that of magnetising the measuring scale alternately, regularly and irregularly, with the lowest possible level of unwanted stray flux during the magnetisation operation so that, in the later operation of detecting the alternate magnetic segments of the measuring scale, the result achieved is a sinusoidal electrical signal which is as precise as possible, and which has as few harmonics as possible.
In this connection, it should be explained (see FIG. 1b) that a sensor which moves at a given spacing above the surface of the measuring structure which has the magnetised segments of differing poles, in the longitudinal direction thereof, detects the gradient, that is to say the first derivative, of the magnetic field lines which extend in a U-shape from each pole to the respectively adjacent poles, and in so doing pass with their free ends into and out of the surface of the measuring structure. Accordingly, the result produced is an electrical signal which is sinusoidal with a zero passage or a reversal in the curvature of the sinusoidal curve at each of the limits between two differently polarised magnetic segments.
For that reason, in regard to the regular sequence of segments, and the irregular segment sequence disposed therein, which are to be used as reference marks, care is to be taken to ensure that those irregular segments are not so long that they achieve double the length of the regularly arranged segments, but are at a maximum only 1.5 times, in particular only 1.3 times, that length (see FIG. 1b).
Based on that situation, the object of the present invention is to provide a process and an apparatus for producing a magnetic measuring scale which satisfies the specified properties in respect of the electrical measuring scale, in spite of the apparatus being of a simple and inexpensive structure and the process being simple and inexpensive to implement.
Due to the L-, C-, U-, F- or 8-shaped configuration of the magnetisation head which acts as a flux guide portion and which in its extent at a location has the effective magnet, a permanent magnet or a separately excitable, in particular electrically excitable, magnet, the band to be magnetised can be introduced into the gap (air gap) of the for example C-shaped configuration of the magnetisation head and thus the magnetic flux can be introduced into the measuring scale to be magnetised, with a scatter effect which is as slight as possible.
In addition magnetic induction into or through the measuring scale can be altered byxe2x80x94mechanical or electricalxe2x80x94influencing of the magnet of the magnetisation head, even without having to move the magnetisation head away from the measuring scale.
To provide for the mechanical influencing effect, it is sufficient to move the magnet out of the alignment of the for example C-shaped flux guide portion, that is to say the remainder of the magnetisation head, or it is sufficient to pivot the magnet so that the pole direction thereof is; no longer aligned with the direction in which the corresponding limb of the magnetisation head extends.
An installation for the magnetisation of such measuring scales includes at least one magnetisation head for magnetisation of the measuring scale and a first and a second motion unit, of which the first moves the measuring scale in the longitudinal direction and the second moves the magnetisation head or a part thereof in order to alter the magnetic flux. Therefore, as described hereinbefore, the second motion unit can displace or rotate the magnet relative to the motion head, but it can also mechanically displace parts of the rest of the magnetisation head, which in fact act as flux guide portions, in particular it can lift them off the measuring scale to be magnetised, by pivotal movement etc. Instead, the second motion unit can also displace the entire magnetisation head in a transverse direction relative to the longitudinal direction of the measuring scale, that is to say it can lift it off the top side of the measuring scale, or it can move it away in a transverse direction, parallel to the direction of the surface, from the measuring scale.
If, upon magnetisation, the measuring scale remains arranged stationarily and only the magnetisation head is moved, the first and second motion units can respectively engage the magnetisation head or parts thereof and in particular can be combined to form a single motion unit.
The magnetisation installation may also have a second magnetisation head so that the regular sequences of differently magnetised segments can be produced with the one magnetisation head and the irregular sequences can be produced with the other magnetisation head.
In that respect the magnetisation head for production of the regular sequences may be in particular a pole wheel which rolls in the longitudinal direction on the measuring scale and which along its periphery has alternately differently magnetised regions.
Another installation serves for thexe2x80x94preferably regularxe2x80x94different magnetisation of a measuring scale which however must be uniformly pre-magnetised, in which therefore as the initial condition for example the top side of the substantially band-shaped measuring scale always forms a North pole and the underside always forms a South pole.
A measuring scale which is pre-magnetised in that way can be polarised in opposite relationship in portion-wise manner by the application of a magnetisation head, For that purpose, a magnetisation head is moved to the corresponding side of the measuring scale; the magnetisation head is of the polarity of the side of the measuring scale, in opposite relationship thereto, and therefore produces there the opposite polarity.
So that this happens in a portion-wise manner, provided between the measuring scale and the corresponding magnetisation head is a cover of non-magnetisable material which has openings only at those locations, that is to say generally at each second longitudinal portion, at which magnetisation reversal of the measuring scale is desired.
As a result of that arrangement, the magnetisation head used can extend over a plurality of and in particular a multiplicity of portions of the measuring scale and in particular can be in the form of a ring pole, that is to say with the same polarity over the entire periphery, which rolls on the surface of the measuring scale. The cover is either arranged stationarily on the measuring scale and is moved therewith relative to the magnetisation head or, in the case of a ring pole, it is arranged concentrically around the pole and rolls together with same on the surface of the measuring scale.
From the point of view of the operating procedure of the process involvedxe2x80x94to afford manufacture which is as inexpensive as possiblexe2x80x94the measuring scale to be magnetised can firstly be produced with the regular sequences of differently magnetised segments, and in that case preferably over the entire desired length of the later measuring scale, that is to say when dealing with portions of the measuring scale which have been previously cut to length, that entire length, preferably in the form of an endlessly manufactured band which is regularly alternately magnetised throughout.
In that respect the magnetisation operation is preferably also effected in a continuous process, preferably by means of an operation involving rolling a pole wheel, which can be effected more quickly than discontinuous, portion-wise production of the individual segments by means of magnetisation, although under some circumstances the latter procedure can afford a better quality in respect of magnetisation of the individual segments.
Then the irregular sequences of segments are applied, more specifically either on previously non-magnetised regions of the measuring scale or preferably by reversal of the magnetisation of the regular markings previously present in those lengthwise regions.
The latter is to be preferred in particular when the irregularly marked regions do not extend over the entire track width but only over a sub-track, in the transverse direction.
The irregular sequences are preferably not applied by means of rolling a pole wheel but they are rather applied by putting a magnetisation head from above on to the top side of the measuring scale or by pushing a C-shaped magnetisation head on to the measuring scale in the transverse direction at the appropriate longitudinal position in order thereby to effect magnetisation.
The magnetic flux into the measuring scale which magnetises it or reverses the magnetisation thereof in a segment in the desired manner should preferably occur only when the magnetisation head is in the desired relative position with respect to the measuring scale.
In order to avoid the need to remove the magnetisation head in the transverse direction on every occasion, that is to say between the operation of magnetising two different segments, and then re-fitting it in the new longitudinal position, but in order to have to effect only a relative displacement in the longitudinal position, then, instead of transverse displacement of the magnetisation head, it is also possible for the magnetic flux to be interrupted in another way:
One possibility: the magnet which is effective in the magnetisation head is a separately excited, in particular electrically excited magnet, the excitation of which can be switched off for a short time in the change in relative position of the magnetisation head.
The other possibility provides that the magnet which is effective within the magnetisation head and which, in the case of a C-shaped magnetisation head, is preferably disposed in the connecting central limb, is so altered in its position that there is no longer any magnetic flux in the magnetisation head, that is to say the magnet is either moved away out of its aligned position in the magnetisation head or it is at least pivoted in such a way that the actual pole direction of that magnet is no longer in conformity with its reference or target pole direction which is generally the direction in which the limb carrying the magnet extends, but differs therefrom in particular through 90xc2x0.
A further possibility involves making the magnetisation head in the shape of an 8, that is to say with three connecting limbs. If in that case the magnet of the magnetisation head is arranged in one limb, and a further connecting limb has the gap (air gap) for passing the measuring scale therethrough, then the remaining third limb can be provided with a removable central region. If the third limb and also its removable region act as a flux guide portion, that is to say comprise material which can be well-magnetised, then that third limb can be activated and deactivated in respect of its function as a flux guide portion, by being removed and moved back again.
When the flux guide portion is activated the ring of the magnetic flux lines is embodied from the magnet by way of the third limb and back to the magnet.
When the flux guide portion in the third connecting limb is deactivated, the ring is embodied by the magnet, the measuring scale in the air gap and back to the magnet, therefore strong magnetic induction occurs in the air gap and thus in the portion of the measuring scale which is disposed there.
In the case of endlessly manufactured, regularly magnetised bands, the operation of cutting the measuring scale to the desired length can be effected optionally prior to or after the application of the irregular sequences of magnetised segments.
A particularly preferred embodiment of a magnetisation installation would therefore firstly provide an endlessly supplied band, by means of a first magnetisation head in the form of a pole wheel in a continuous process, with a regular continuous sequence of magnetised segments which thereafter, in the direction of travel of the band through the installation, are provided with the irregular sequences of segments by means of magnetisation reversal at the desired longitudinal positions by means of a second, discontinuously operating magnetisation head, and thereafter it is cut to length at the necessary locations.